(1) Field of the Invention
The invention relates to active microoptic reflecting components for adapting or changing the focal distance or focal position in optical systems.
(2) Description of Related Art
In the optical detection of measuring variables or data, the position of the focal point must in many cases be varied or the lack of a variation option restricts the performance of the measuring-detecting system. For example, the confocal measuring principle may be cited in the following, in which a variation in the focal position is used, and reading of barcodes with a laser scanner in which the lack of a focus variation leads to a restriction in performance.
a) Measuring system which is based on a variation of the focal position: the confocal measuring principle is used principally for measuring the surface topology of an object. The focal point must be varied perpendicular to the surface (z direction) for this purpose. This is frequently achieved such that the optical path length from the point light source to the object is changed continuously or progressively. Due to the lenses which are disposed suitably in the optical beam path, the position of the focal point is consequently varied.
b) Optical system in which the lack of a focal variation leads to a restriction in performance: when reading barcodes the position of the focal point determines the location at which barcodes with the highest density can be read. The greater the spacing between barcode and focal point, the smaller is the resolution of the scanner. Barcodes of high density can therefore only be detected within a very restricted reading distance. The performance of the scanner is hence limited.
In order to vary the focal distance or position of the focal point with the boundary condition that both the measuring system and the object to be measured/detected are not moved, there are two approaches in principle.
The first approach is based on extending the optical path length between light source and outlet aperture of the light beam from the measuring system. For this purpose, in the simplest case a planar mirror is moved perpendicularly to the incident light beam. The movement of the mirror requires a drive which can be based for example on an electromagnetic principle. In the case of precision-engineering production of such a mirror with a drive, the result is relatively large constructional volumes and high costs. For mobile applications such as scanner guns, this method is thus not suitable. Micromechanically manufactured translatory mirrors are characterised by relatively small movements so that the achievable optical path length changes are comparatively small. In order to convert this small path length change into a significant change of position of the focal point, a complex lens system would be required which is very intolerant in relation to position change of the optical components (tolerances in construction, thermally induced position changes etc.).
The second approach is based on using deformable mirrors. The deformation is thereby chosen such that a hollow mirror with an adjustable focal distance is produced. The mirror deformation is achieved via actuators which are situated underneath the mirror. In the case of systems manufactured using precision engineering, as are used in astronomy for adaptive-optical systems, piezoactuators are used in general. Such systems are very complex, expensive and have a large volume. Micromechanically manufactured, deformable mirrors are smaller and cheaper. Here, the static or quasistatic deformation is generally generated by electrostatic forces. Either arrays comprising individual, translatory and possibly additionally rotationally adjustable mirrors or membrane mirrors are used here. The arrays have the disadvantage that, as a result of segmentation of the reflective surface, diffraction effects occur which significantly reduce the beam quality. Membrane mirrors comprise a continuous mirror membrane which is generally deformed by actuators which are disposed in an array under the membrane. Almost without exception, electrostatic or piezoelectric actuators are used for this purpose. Both variants have the disadvantage that high electrical voltages are required for deformation of the membrane.